1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing or increasing a hydraulic braking pressure supplied to a wheel brake cylinder. In view of the fact that when the hydraulic braking pressure supplied to the wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before a coefficient of friction of a road surface relative to the road wheel reaches a maximum, the anti-skid control system controls the hydraulic braking pressure in response to the variation of wheel speed and that of wheel acceleration in order that a slip rate of the road wheel results in around 10% to 20%, that is, the maximum coefficient of friction is obtained.
According to the anti-skid control system as described above, in general, a rotational speed of each of the road wheels or a wheel speed is detected, and an estimated vehicle speed is calculated on the basis of the wheel speed to provide a threshold value which is compared with the wheel speed of each road wheel, and then the hydraulic braking pressure in a wheel brake cylinder mounted on each of the road wheels is controlled in response to the result of comparison. And, Japanese Patent Laid-open Publication No. 64-52568, it has been proposed to improve such anti-skid braking system that provides a "decrease" zone, or a zone for decreasing the hydraulic braking pressure in the wheel brake cylinder, and an "increase" zone, or a zone for increasing the hydraulic braking pressure, in response to the slip rate and the wheel acceleration, and that can vary the extent of each zone. In the above publication, there is disclosed a system which enables the memory to reduce its capacity to be needed.
However, in the above-described prior system which simply provided the decrease zone and increase zone separately, the condition for controlling the braking force is likely to vary due to the difference in decrease-increase characteristic of pressure control means for controlling the hydraulic braking pressure of every vehicle, the difference in size of the tire, the difference in characteristic of the vehicle braking system as a whole, or the difference in coefficient of friction of the road surface. Although the above-described prior system may provide a pulse decrease control which alternately performs a "decrease" operation for decreasing the hydraulic braking pressure and a "hold" operation for holding the hydraulic braking pressure as it is in the decrease zone, it will be difficult to adequately control the hydraulic braking pressure when the proportion of the decreasing time and the holding time has been fixed in advance.